doc: clarify use of Uint8Array for n-api

`napi_get_buffer_info` always supported receiving `Uint8Array` as a
`value` argument because `node::Buffer` is a subclass of `Uint8Array`
and the underlying V8 APIs don't distinguish between two. With this
change we mark both types as supported by the API so that the user code
doesn't have to unknowingly use oficially unsupported type of the
`value` argument.

doc/api/n-api.md

@@ -3122,9 +3122,10 @@ napi_status napi_get_buffer_info(napi_env env,
 ```
 
 * `[in] env`: The environment that the API is invoked under.
-* `[in] value`: `napi_value` representing the `node::Buffer` being queried.
-* `[out] data`: The underlying data buffer of the `node::Buffer`.
-  If length is `0`, this may be `NULL` or any other pointer value.
+* `[in] value`: `napi_value` representing the `node::Buffer` or `Uint8Array`
+  being queried.
+* `[out] data`: The underlying data buffer of the `node::Buffer` or
+  `Uint8Array`. If length is `0`, this may be `NULL` or any other pointer value.
 * `[out] length`: Length in bytes of the underlying data buffer.
 
 Returns `napi_ok` if the API succeeded.